The present invention relates to a laser scanning system employed in an electrophotographic imaging apparatus such as a laser beam printer and the like.
Conventionally, the optical scanning systems, such as the laser scanning system, have been employed in a laser beam printer. In such laser scanning systems, for example, a laser beam emitted by a laser emitting device is deflected by a polygon mirror and directed to the surface of photoconductive member through the f.theta. lens. As the polygon mirror rotates, the laser beam is deflected by the light reflecting surface of the polygon mirror, and repeatedly scans a predetermined angle zone of the surface of the photoconductive member. The laser beam is ON/OFF modulated in accordance with an image to be processed. In order to synchronize the scanning operation, as described above, with the imaging operation, the deflected laser beam is received at a predetermined position before the deflected laser beam scans the predetermined angle zone. For this purpose, a light receiving member is provided at an upstream side with respect to the scanning direction of the laser beam. Further, in order to let the deflected beam be focused on the light receiving member, a converging lens is provided between the polygon mirror and the light receiving member. The light receiving member outputs a signal upon receiving the laser beam. Thus a horizontal synchronous signal is generated by the light receiving member. When a predetermined period elapses, after the horizontal synchronous signal is generated, the laser beam scans the predetermined angle zone of the photoconductive member as aforementioned.
If the position of the light receiving member is shifted, since the scanning operation is performed at a predetermined period after the horizontal synchronous signal has been generated, the laser beam can not exactly scan the predetermined angle zone of the photoconductive member. In other words, the light receiving member should be accurately installed in the laser scanning system.
Further, in the construction described above, in general, the focal length of the lens, disposed between the polygon mirror and the light receiving member, is made smaller than that of the f.theta. lens. Accordingly, only a small amount of difference in the position of the light receiving member affects the scanning operation.